Dyeing of cellulosic textile materials by forming lakes of basic dyes in the fibers



Patented Mar. 18, 1952 DYEING OF CELLULOSIC TEXTILE MATE- RIALS BY FORMING LAKES F BASIC DYES IN THE FIBERS William Stuart Miller, Manchester, England, as-

nr OFFICE signor to The Calico Printers Association Limited, Manchester, England, a British company No Drawing. Application July 3, 1950, Serial No. 172,010. In Great Britain June 24, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires June '24, 1963 16 Claims.

usually steaming to promote penetration and then acidifying to precipitate the resulting insoluble lake inside the textile fibers. The invention also includes the dyed fabric produced antimony mordants but with greater light fastness. Attempts have also been made to reduce the number of operations required in the dyeing operation by premixing the dyestufi with various mordants, under conditions preventing the precipitation of the lake, followed by treating the tixtile with the mixture of dye and .mordant.

One proposal has been made to mix .a basic dye with excess caustic alkali to form the free mordant, the free caustic alkali present being relied upon to prevent precipitation of the lake until after application to the fabric. Another proposal is to dissolve the dye and the mordant in a large excess of organic solvent followed by treating the fabric with the solution and precipitating the lake on the fabric by a water treatme'ht. Various other expedients have been suggested, such as preliminary fixation of the dye with tannic acid followed by after-treatment with a lake-forming inorganic acid, or application of the lake-forming acid to the fabric first, followed by application of the dyestuif .in a separate operation. But all of these prior as described; all as more fully hereinafter set processes, apart from being complicated or exforth and as claimed. pensive, have given dyed products seriously,

This is a continuation-in-part of my copcndlacking in brightness, fullness and fastness to ing application, Serial No. 761,737, filed July washing and rubbing because in all or practically 17, 1947, and now abandoned. In my prior apall cases the lake has been deposited on the plication the same general dyeing process is de- :0 surfaces rather than inside the fibers. While scribed and claimed. In the present case the some of these methods are capable of producing conditions required to produce the best results dyed fabrics having a fastness to light better in the process are more closely defined and the than that obtained with the conventional tanprocess is exemplified by several aditional spcmin-antimony mordant, the colors produced have cific examples. 15 been so inferior in brightness that none of these It has long been known that basic dyestuffs methods has come into commercial use or supercombined with various lake-forming inorganic ceded the conventional method. acids, such as phosphotungstic and phospho- I have now discovered what I believe to be molybdic acids to form colored lakes which have the first practical and simple method of dyeing good fastness 130 light- Preformed pig ts with basic dyestuffs and lake-forming inorganic formed in this manner have been used to a acids with the production of dyed fabrics whose limited extent in the printing of textile fabrics colors are at least as bright as those produced by the conventional method of mechanically with tannin-antimony mordants combined with fixing the pigments to the textile by means of an full color yield and extreme fastness to light, to adhesive. .Such prints, however, suffer from the washing and to rubbing. Although the dyestuff de c Common to most P ts pp d in this and mordant are applied to the fabric in admanner, namely they are not fast to washing mixture surprisingly my tests have shown that or rubbing. And they also unduly stiffen the the insoluble lakes are precipitated within the fabric. fibers rather than between and on their surfaces Tannin-antimony mordants are also used to and it is to this result that I attribute most of fix'basic dyestuffs. This method produces colors the superior properties of the dyed products. of extreme brightness but, unfortunately, these The process itself is simple, expeditious and incolors are not light fast. Many attempts have expensive and the coloring matter is applied to been made to develop a method giving colors the textile material together with the lake-formhaving the brightness of those fixed with tannining acid component in a single operation. This appears to be the first time that a dyestuff and a mordant have been applied to a fabric in admixture followed by precipitation of the lake inside the fibers.

I have discovered that, if basic dyes are dissolved in a substantially aqueous medium and admixed with inorganic lake-forming acids under conditions producing a pH within the critical range of from 5 to 12.5, precipitation of an in-- soluble lake is prevented and the mixture can.

be applied to a textile to be dyed followed by acidification-to produce precipitation of the lake in situ. The lake-forming acid is advantageously added to the dye solution in the form of its alkali metal salt dissolved in water. It is desirable to have a buffering salt present in order to ensure maintainance of the pH within the range stated during the dyeing procedure and prior to the acidification step.

The substantially aqueous medium in which the dye is dissolved may be water alone or it may be water with an admixture of a water-miscible nonacidic or neutral organic solvent for the dyestufi in an amount which need not materially exceed 15% by weight of the complete dyeing or printing mixture. These solvents are merely used to facilitate the dissolution of the dyestufi in such cases where the dyestuff is difficult to dissolve in water alone. In the proportions used they have no effect whatever on the formation of the color lake.

, Suitable water-soluble non-acidic organic solvents are for instance methyl and ethyl alcohols, tetrahydrofurfuryl alcohol, ethylene glycol, ethylene glycol monomethyl and monoethyl ethers, diethylene glycol, diethylene glycol monoethyl ether and thiodiglycol, but my invention is not limited to the use of these particular solvents, since any liquidorganic solvent for the dyestufi can be used which is water-miscible, inert toward the components of the printing mixture and substantially neutral in aqueous solution. Mixtures of solvents may also be used.

' Among the lake-forming inorganic acids which have been used or suggested in forming lakes with basic dyestuffs, I have found phospho-molybdic acid to be by far the most suitable in my process. It is used in the form of its alkali metal salts and the sodium salt is preferred on the grounds of cost and availability.

I may use sodium phospho-molybdate, or I may add its components, i. e. sodium molybdate, and phosphoric acid or an alkali phosphate, directly to the dyeing solution or printing paste, when phospho-molybdate will be formed prior to or during the lake-forming reaction. Thus 'all possess thecommon property of containing a They may be. weak acids. derived from transition metals, such asqn'olybdic' and tungstic acids, or complex hettransition' metal in their anions.

eropolyacids in which the nuclear acid is, for example, phosphoric, arsenic or silicic acid and the coordinated acid is one of the mentioned weak acids.

In order to get the best color yield it is essential to prepare the dyeing solution or printing 'paste composition at a pH between 5 and .12.5. If lower than 5 the color lake may be wholly or partially formed before impregnation of the cloth thus. defeating one. of the .main objects of my process. If above 12.5 the strongly alkaline nature of the composition tends'to destroy the basic dyestuff at some stage, particularly during the steaming process, prior to precipitation of the color lake with a consequent loss in color. Any

serious loss in final color yield produced in this manner would of course make the process commercially useless.

The buffering of the dyeing or printing composition to a pH between 5.0 and 12.5 may be effected with any suitable buffer, but alkali phosphate buffers are preferred not only. because they cover the desired range easily but also because they can serve in the formation of the required phospho-molybdate as well. Any compatible non-reactive salt capable of buffering aqueous solutions within the required pH range is suitable. Citrate and borate buffers can be used, for example.

It will be seen from the above that the dyeing or printing compositions used in my invention are composed of three essential parts, i. e. the solution of the basic dyestuff, the solution of the phospho-molybdic mordant, and the buffer, although the buffer can sometimes be dispensed with if the mordant is used in the form of its alkali metal salt and this provides a suflicient buffering effect. The order of mixing may be varied at will, provided always that when the dyestuff and mordant solutions meet, a pH not less than 5 is obtained. It is possible for instance, to mix the mordant solution with the buffer, and stir this mixture into the dyestuff solution, or to pour the dyestufi solution into the mixture of buffer and mordant. It is also possible to mix the dyestuff solution with the buffer first, and then add the mordant solution, or vice versa.

'- 'Printing compositions naturally also contain a suitable thickener, such as gum tragacanth or carob bean gum. The thickening may be mixed with the dyestuff, buffer and/or mordant solutions or various combinations of same in any convenient manner always provided that dyestuff and mordant do not meet under conditions giving a pH less than 5. Hygroscopic agents such as glycerol or urea may also be added. Choice of suitable buffering agents, thiEkeners and hygroscopic agents are well within the skill of the art.

The compositions so prepared are stable indefinitely. They may be applied to all cellulosic textiles by any known method, such as padding, machine or screen printing, stenciling and the like, but they are of particular interest in the printing of regenerated cellulose rayon fabrics, where the brightness of the colors shows to its best advantage.

After padding or printing, the fabric is dried and advantageously steamed to assist penetration of the dye and to ensure that on subsequent acidification the lake will form within the fiber. It is then acidified by immersion in an acid solution or by exposure to an acid atmosphere. Any organic acid as strong or stronger than formic acid may be used to make the acid solution; mineral acids such ashydrochloric or sulphuric acids may also be used, provided always that the conditions are such as not to attack the fabric or the dyestuffs. The acid solution may be used cold or at elevated temperature. If it is preferred to pass the printed fabric through an acid atmosphere, formic acid or a mixture of formic and acetic acids are suitable, but acetic acid alone is likely to give inferior results; Such an acid atmosphere is preferably maintained at an elevated temperature and may be obtained by mixing acid vapors and steam as is normally practiced in the usual acid ageing process-of calico printing.

The following represent practical operating examples ofv my process which exemplify various methods and materials which can be employed in the production of my novel dyed fabrics.

Example 1 1 g. Victoria Blue (Col. Ind. No. 728) is dissolved in a mixture of '8 g. ethyl alcohol, 30 g. water and g. urea (which latter acts as a hygroscopic agent in the subsequent steaming), and the resulting solution is stirred into carob bean thickening. An aqueous solution of 3 g. sodium molybdate which has been adjusted to a pH between 9 and 11 is then added and the mix-' ture is bulked to 100 g. with carob bean thickening. The paste, containing 8% ethyl alcohol, is printed on a viscose rayon .crepe fabric, and after drying, the fabric is steamed for one hour to assist penetration of the still soluble dyestuff. The insoluble lake is then precipitated in the fiber by ashort passage through steam and acetic and formic acid vapors and the fabric is finally soaped, rinsed and dried.

Errample 2 2 g. Xylene Blue AS (Col. Ind. No. 673) are dissolved in water with the addition of 9 g. ethylene glycol monoethyl ether and 5 g. glycerol (which latter acts as a hygroscopic agent in the subsequent steaming). 1.4 g. disodium hydrogen phosphate and 0.25 g. sodium dihydrogen phosphate, both previously dissolved in water, are now added and the whole stirred into tragacanth thickening. Finally 3 g. sodium phosphomolybdate in aqueous solution of a pH between 9 and 11 are added and the mixture bulked to 100 g. The paste, containing 14% of glycerol plus ethylene glycol monoethyl ether, is printed on a spun viscose fabric which after drying is steamed for 20 mins. then passed through steam and acetic and formic acid vapors, washed and dried.

Example 3 1 g. of Brilliant Green Crystals YS (Col. Ind. r

No. 662) is dissolved in water together with 10 g. diethylene glycol monoethyl ether. The resulting solution is stirred into tragacanth thickening; and addition is made of 5 g. sodium phosphotungstate, previously dissolved in water and adjusted to apH between 9 and 11, and the whole bulked to 100 g. A cotton cambric is printed win with this paste, containing 10% diethylene glycol monoethyl ether, dried, passed through steam and acetic and formic acid vapor, washed and dried.

Example 5 "mixed fabric constructed of cotton warp and '6 viscose filament weft. After drying, the printed fabric .is steamed for '3 mins., passed through steam and acetic and formic acid vapor, washed and dried.

Example 6 1 g. of -Brilliant Green Crystals YS (Col. Ind. No. 662) is .dissolved in ml. water by boiling and the resultant solution stirred into gum tragacanth thickening. '3 g. of sodium molybdate, 1.4 g. of disodium hydrogen phosphate and 0.25 g. of sodium dihydrogen phosphate are dissolved together in water and added to the thick ened dyestuif, the mixture being finally bulked to 100 g. The resulting printing paste was a pH of 6.95. It .is printed onto a viscose rayon fabric, which after dryingis steamed, passed through steam .and formic acid vapors to bring about lake formation, and finally washed and dried.

Example 7 .3 .g. of sodium molybdate, 1.4 g. of disodium hydrogen phosphate and 0.25 g. of sodium dihydrogen phosphate are dissolved together in water and the resultant solution stirred into gum tragacanth thickening. 1 g. of Victoria Pure Blue BO the hydrochloride of pentaethyl-triaminodiphenyl-l-naphthyl-carbinol anhydride) is dissolved in water with the addition of 5 g. tetrahydrofurfuryl alcohol and 5 g. glycerin and this solution is added to the above paste, which is then bulked to 100 g. The paste, which has a pH of 7.1, is printed on to a viscose rayon fabric, which after drying is steamed for 40 min. The lake is then precipitated by immersing thesteam- 1 ed print in Tw: hydrochloric acid at room temperature for 5 mins., after which it is rinsed free of acid, soaped, rinsed and dried.

Example '8 A solution is made of 1 g. Victoria Pure Blue BO, 5 g. urea, 10 g. diethylene glycol monoethyl ether and 15 g. water. A second solution of 1.4 g. disodium hydrogen phosphate and 0.25 g. sodium dihydrogen phosphate in 20 g. water is then mixed with it and the whole stirred into 30 g. of tragacanth thickening, after which a solution of 3 g. sodium molybdate in 15 ml. of water is finally added. The pH of the mixture is 7.1. The paste is printed on to a cuprammonium rayon fabric, dried and steamed for 40 mins. The lake is then'precipitated by immersion in aqueous formic acid at C. for 2 mins. The fabric is finally rinsed free from acid, soaped, rinsed and .dried.

Esample 9 0.05 g. of Victoria Pure Blue B0 is moistened with 1 g. ethyl alcohol and brought into solution by the addition of 20 g. of boiling water. The resulting solution is stirred into 40 g. of tragacanth thickening and then a solution of 3 g. sodi- "um molybdate and 0.6 g. phosphoric acid in 20 m1. of water added finally. The pH of the mixture is 6.7. The paste is printed onto a viscose rayon fabric, dried, steamed for 20 mins. and the lake precipitated by immersion in /2 Tw. hydrochloric acid for 2 minsxat 55 C. The fabric is then after-treated as in previous examples.

Example 10 1 g. of Rhodam'ine 6G extra (Color Index No. 752) is dissolved in water with the assistance of 10g. of ethylene glycol monoethyl ether, and the solution is stirred. into tragacanth thickening. .A solution of 3 g. sodium *molybdate and 1.5 g. phosphoric acid is then added and the paste 7 bulked with water to 100 g., the resulting pH being 5.8. The paste is printed on a spun viscose rayon fabric, which is then dried,1steamed for 1 hr. and acidified in the vapors of acetic and formic acids, after which the print is washed off as in Example 6. l

. Example 11 1 g. of Brilliant Green Crystals YS (Color Index N0. 662) is dissolved in water with the addition of 4 g. ethyl alcohol and the solution stirred into carob Bean thickening. A solution of 3 g. of sodium molybdate and 1.2 g. of phosphoric acid is added and the whole bulked with water to 100 g. having a pH of 6.1. A viscose rayon fabric is evenly impregnated on the padding mangle with the above thickened solution and dried. The cloth is then further processed as in Example '7.

Example 12 1 g. of Victoria Pure Blue B is dissolved in water with the assistance of g. of ethylene glycol. A solution of 3 g. of sodium molybdate and 0.9 g. phosphoric acid is stirred into tragacanth thickening and the dye solution previously prepared is then added, the paste being finally bulked to 100 g. of a pH of 6.8. The paste is applied to a viscose rayon fabric by printing, and after drying and steaming for mins. the fabric is soured and after-treated as in Example 9..

Example 13 Example 14 1 g. of Victoria Pure Blue B0 is dissolved in water with the addition of 5 g. diethylene glycol monoethyl, ether, and the solution stirred into tragacanth thickening. A solution of 3 g. sodium phospho-molybdate; 1.4 g. of disodium hydrogen phosphate and 0.25 g. of sodium dihydrogen phosphate'is added and the paste bulked with water to 100 g. having a pH of 6.9. A mercerized cotton sateenis printed with the resulting paste, dried, steamed for mins., then treated in aqueous formic acid at the boil for 2 mins. and finally after-treated as described before.

Example 15 Example 16 3 g.of sodium molybdate and 2.2 g. of trisodium phosphate are dissolved in water and stirred into tragacanth thickening. 1 g. of Rhodamine 6G extra (Color. Index No. 752115 dissolved in 5 g. of diethylene glycol monoethyl ether and 20 g. of boiling water and the resulting solution stirred 8 into the paste which is then-bulked to 100 g. with water. The pH is 11.3. The paste is printed on a cuprammonium rayon fabric which is then dried and further processed as in Example 6.

v Example 17 l g. of Brilliant Green Crystals YS is dissolved in water-with the aid of 4 g. of methyl alcohol and the solution is stirred into tragacanth thickening. An aqueous solution of 3 g. of sodium molybdate, 0.55 g. of trisodium phosphate and 1.5 g. of disodium hydrogen phosphate is then added and the paste bulked to 100 g., the resulting pH being 7.7.

This paste is then printed on a viscose rayon fabric which is dried and processed as in Example 10.

Example 1 8 1 g. of Victoria Pure Blue B0 is dissolved in water with the aid of 5 g. of ethylene glycol monoethyl ether and the solution is pasted into tragacanth thickening. A solution of 3 g. of sodium molybdate, 2.2 g. of trisodium phosphate and 0.4 g. of sodium hydroxide is then added and the whole bulked to g. The pH is 12.2. The'paste is printed onto a viscose fabric which is then dried and further processed as in Example 14.

' Example 19 1 g. of Brilliant Green Crystals YS (Colour Index No. 662) is dissolved in water with the aid of 5 g. of tetrahydrofurfuryl-alcohol and the resulting solution stirred into tragacanth thickening. A solution of 3 g. of sodium phospho-molybdate, 1.1 g: of citric acid and 0.61 g. of sodium hydroxide is then added and the Whole bulked with water to 100 g. of pH=6.6. A viscose rayon fabric is printed with the paste and after drying is processed as in Example 6.

While I have described what I consider to be the best embodiments of my invention it is obvious, of course, that many variations can be made in the specific procedures which have been described without departing from the purview .of this invention, provided that the principles outlined above are followed in the selections.

Conventional procedures and materials can be used to a large extent. For example the alkali which must be present in the mixed aqueous solution of dyestuff and inorganic lake-forming acid to produce a pH of from 5 to 12.5 and to prevent precipitation of the lake can be any alkali which remains soluble throughout the process. This alkali, of course, is usually added either to an aqueous solution of the lake-formin acid to produce the corresponding salt or a pH advantageously within the range of about 9 to 11, for example, or to an aqueous solution of the dyestuff or partly to both prior to mixing of the two solutions to make the mixed lakeforming solution or paste. If a buffer is used this can be added either to dyestufi solution or the lake-forming acid solution prior to admixture of these solutions or under certain conditions it can be added to the final mixture. The only requirement is that the alkali and/or buffer beadded or present in the mixture soon enough so that precipitation of the lake is prevented.

The essential steps in my process are (1) the preparation of a mixed buffered aqueous solution of a basic dye and of a lake-forming inorganic acid (containing a transition metal in precipitation of the lake with sufficient buffer being present to hold the solution within this range during application to the fiber (3) impregnating the fiber with said mixed solution, (4) steaming of the impregnated fiber (essential for best results) to produce penetration of the dye into the fibers and (5) acidification of the dye on the fiber with an acid at least as strong as formic acid but not strong enough to damage the fiber. Photomicrographs taken across fibers dyed by this method have demonstrated that the lake is actually precipitated inside rather than on the outside of the fiber. Photomicrographs taken in color show this in most striking fashion. I believe this is the first time that this result has been accomplished with a mixture of basic dyes and inorganic lake-forming acids of the type defined. As indicated previously the colors produced are at least as bright as those produced with the usual tannin-antimony mordants. The colors are substantially as light fast as the colors ofthe lake pigments. And, owing to the fact that the lake is formed inside the fibers, the colors are also fast to washing and rubbing. In addition my process has the advantages of producing full color yield and of being simple, practical and inexpensive. Further modifications of my process which fall within the scope of the following claims will be immediately evident to those skilled in this art.

What I claim is:

1. In the dyeing of cellulosic textile fibers, the process which comprises preparing an aqueous mixed solution of a basic dyestuff and a lakeforming inorganic acid with sufilcient alkali present to produce a pH within the range of from 5 to 12.5 and to prevent precipitation of the lake; the mixed solution being buffered sufiiciently to maintain the pH within said range during application of the solution to the fiber; applying the mixed solution to the fiber to be dyed and acidifying with an acid at least as strong as formic acid to cause precipitation of the lake within the fiber.

2. The process of claim 1 wherein the said mixed solution contains up to about per cent of a water-miscible non-acidic organic solvent for the dyestuff to facilitate dissolution of the latter.

3. The process of claim 1 which includes the step of steaming the dye impregnated fiber to increase the penetration of the dyestufi' into the fiber.

4. The process of claim 1 wherein the lakeforming acid is a complex acid in which the nuclear acid is selected from the group consisting of phosphoric, arsenic and silicic acids while the coordinated acid is selected from the group consisting of molybdic and tungstic acids.

5. The process of claim 1 wherein the aqueous mixed solution contains a thickening agent and a hygroscopic agent.

6. The process of claim 1 wherein the aqueous mixed solution is prepared by mixing an aqueous solution of the dyestuif with an aqueous solution of the alkali metal salt of the lake-forming acid.

7. The process of claim 6 wherein the aqueous solution of the dyestuff contains a water-miscible non-acidic organic solvent for the dyestuif suflicient to solubilizethe latter and amounting to not more than 15 per cent of the solvent based on the weight of the aqueous mixed solution.

8. The process of claim 1 wherein a buffering alkali phosphate is present in the aqueous mixed solution.

9. The process of claim 8 wherein the buffering salt is added to an aqueous solution of the lakeforming acid prior to mixing the latter with an aqueous solution of the dyestufi to form said aqueous mixed solution.

10. The process of claim 1 wherein said aqueous mixed solution is formed by mixing an aqueous solution of the dyestuif containing a buffering salt with an aqueous solution of the lakeforming acid in the form of its alkali metal salt.

11. The process of claim 1 wherein the lakeforming acid is phospho-molybdic acid.

12. The process of claim 1 wherein the fiber is subjected to the action of formic acid vapors to precipitate the lake in the fiber.

. 13. In the dyeing of cellulosic textile fibers, the process which comprises dissolving a basic dyestuff in an aqueous medium containing a suflicient quantity of a water-miscible, nonacidic solvent to solubilize the dyestulf, preparing an aqueous solution of a neutral alkali metal salt of a lake-forming inorganic acid, mixing the two solutions in the presence of an inorganic buffering salt capable of maintaining the pH of the mixture within the range of 5 to 12.5, whereby precipitation of the lake is prevented, applying the mixture to the fiber to be dyed, steaming to increase the penetration of the dye into the fiber and acidifying with an acid at least as strong as formic acid to produce precipitation of a lake inside the fiber.

14. In the dyeing of cellulosic textile fibers, the process which comprises dissolving a basic dyestufi in an aqueous medium, preparing an aqueous solution of sodium phosphomolybdate, mixing the two solutions in the presence of sufficient sodium phosphate to buffer the mixture within the pH range of from 5 to 12.5, whereby precipitation of a lake is prevented, applying the mixture to a fiber to be dyed, steaming to promote penetration of the dyestuff into the fiber and then acidifying with an acid at least as strong as formic acid to precipitate the resulting lake inside the fiber.

15. In the dyeing of cellulosic textile fibers, the process which comprises dissolving a basic dyestuif in an aqueous medium, preparing an aqueous solution of an alkali metal salt of an inorganic lake-forming acid mixing the two solutions in the presence of an inorganic buffer salt capable of maintaining the mixture at a pH within the range of from 5 to 12.5, adding a thickener and a hygroscopic agent to make a printing paste, printing the paste on a fabric to be dyed, steaming to promote penetration of the lake-forming mixture into the fibers and acidifying with an acid of substantially the same strength as formic acid to precipitate the resulting lake inside the fibers.

16. The process of claim 15 wherein a watermiscible non-acidic organic solvent is added to the solution of said dyestuff to promote solution thereof in quantity amounting to not substantially more than about 15 per cent by weight of the mixture of solution.

WILLIAM STUART MILLER.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 485,891 Germany Nov. 12, 1929 522,591 Great Britain June 21, 1940. 566,258 Great Britain Dec. 20, 1944 

1. IN THE DYEING OF CELLULOSIC TEXTILE FIBERS, THE PROCESS WHICH COMPRISES PREPARING AN AQUEOUS MIXED SOLUTION OF A BASIC DYESTUFF AND A LAKEFORMING INORGANIC ACID WITH SUFFICIENT ALKALI PRESENT TO PRODUCE A PH WITHIN THE RANGE OF FROM 5 TO 12.5 AND TO PREVENT PRECIPITATION OF THE LAKE; THE MIXED SOLUTION BEING BUFFERED SUFFICIENTLY TO MAINTAIN THE PH WITHIN SAID RANGE DURING APPLICATION OF THE SOLUTION TO THE FIBER; APPLYING THE MIXED SOLUTION TO THE FIBER TO BE DYED AND ACIDIFYING WITH AN ACID AT LEAST AS STRONG AS FORMIC ACID TO CAUSE PRECIPITATION OF THE LAKE WITHIN THE FIBER. 